1. Field of the Invention
The present invention relates to the field of semiconductor manufacturing. More specifically, it relates to an improved reticle design for ensuring that both the 0 degree light and the 180 degree light passing through an alternating phase shift mask are of approximately equal amplitude.
2. Description of the Related Art
Lithography processing is a required technology when manufacturing conventional integrated circuits (IC""s). Many different lithography techniques exist and all lithography techniques are used for the purpose of defining geometries, features, lines or shapes onto an IC die or wafer. In general, a radiation sensitive material, such as photoresist, is coated over a top surface of a die or wafer and is patterned using lithography techniques to selectively permit the formation of the desired geometries, features, lines, or shapes.
One known method of lithography is optical lithography. The typical optical lithography process generally begins with the formation of a photoresist layer on the top surface of a semiconductor wafer. A mask typically of quartz and having fully light non-transmissive opaque regions, which are usually formed of chrome, and fully light transmissive clear regions (non-chromed) is then positioned over the aforementioned photoresist coated wafer. Light is directed onto the mask via a visible light source or an ultra-violet light source. This light passes through the clear regions of the mask and exposes the underlying photoresist layer, and is blocked by the opaque regions of the mask, leaving that underlying portion of the photoresist layer unexposed. The exposed photoresist layer is then developed, typically through chemical removal of the exposed/non-exposed (depending on whether the photoresist is positive or negative photoresist) regions of the photoresist layer. The end result is a semiconductor wafer coated with a photoresist layer exhibiting a desired pattern. This pattern can then be used for etching underlying regions of the wafer.
In recent years, there has been great demand to increase the number of transistors on a given wafer area. Meeting this demand has meant that IC designers have had to design circuits with smaller minimum dimensions. However, it was found that the traditional optical lithography process placed real limits on the minimum realizable dimension due to diffraction effects. That is, light shining onto the mask is modified as it passes through the mask and therefore the desired photoresist pattern differed somewhat from the pattern actually achieved. In particular, as the minimum dimension approaches 0.1 microns, traditional optical lithography techniques will not work very effectively.
One technique that has been used to realize smaller minimum device dimensions is called phase shifting. In phase shifting, the destructive interference caused by two adjacent clear areas in an optical lithography mask is used to create an unexposed area on the photoresist layer. This is accomplished by making use of the fact that light passing through the clear regions of a mask exhibits a wave characteristic such that the phase of the amplitude of the light exiting from the mask material is a function of the distance the light travels in the mask material. This distance is equal to the thickness of the mask material. By placing two clear areas adjacent to each of other on a mask, one of thickness t1 and the other of thickness t2, one can obtain a desired unexposed area on the photoresist layer through interference. By making the thickness t2 such that (nxe2x88x921)(t2) is exactly equal to xc2xd xcex, where xcex is the wavelength of the exposure light, and n is the refractive index of the material of thickness t1, the amplitude of the light exiting the material of thickness t2 will be 180 degrees out of phase with the light exiting the material of thickness t1.
Since the photoresist material is responsive to the intensity of the light, and the opposite phases of light cancel where they overlap, a dark unexposed area will be formed on the photoresist layer at the point where the two clear regions of differing thicknesses are adjacent. Phase shifting masks are well known and have been employed with various configurations. The configuration described above is known as an alternating phase shift mask (APSM). APSMs have been shown to achieve dimension resolution of 0.25 microns and below.
Referring to FIG. 1, a standard reticle design 100 for an alternating phase shift mask is depicted. The reticle contains a light transmissive region 104 (e.g., quartz) and light non-transmissive regions 102 (e.g., chrome). The reticle 100 also contains a 0 degree channel 116 and a 180 degree phase shift channel 114. As described above, the 180 degree phase shift channel 114 is created by forming an etched portion 112 in the light transmissive region 104 such that the distance traveled within the reticle 100 is shorter for light rays 106 than for light rays 108, where the light rays 106, 108 are provided by illumination source 110.
As described above in connection with alternating phase shift masks, the difference in the distance traveled by light rays 106, 106 through light transmissive region 104 in combination with the refractive index of the light transmissive region 104 and the wavelength of the light rays 106, 108 are calculated such that the amplitude of the light rays 106 exiting the reticle 100 are 180 degrees out of phase with the light rays 108 exiting the reticle 100.
While APSMs have been used with a great degree of success, they still have some drawbacks. For instance, currently, the amplitude of the 0 degree and the 180 degree phase shifted intensities exiting an alternating phase shift mask are not equal. For example, FIG. 2 depicts image intensity for both 0 degree light and 180 degree phase shifted light. It is clear from the graphical representation that the intensity of the 180 degree phase shifted light is substantially less (e.g., approximately 19% less) than the 0 degree light. The reason for this discrepancy is that the first order of diffracted light (designated +1 and xe2x88x921) traveling through the 180 degree phase shift channel 114 does not completely pass through the reticle 100 because it is prevented from doing so by the light non-transmissive regions 102 (e.g., the chrome regions). Since the intensities of the 0 degree and 180 degree phase shifted light rays are not equal, the above-described process of canceling out opposite polarities of light is not optimized. As a result, the unexposed areas of the photoresist where the two light rays overlap are poorly defined, thereby inhibiting optimal IC designs. Thus, a method and apparatus for producing 0 degree light and 180 degree phase shifted light having substantially equal intensities as that light exits an alternating phase shifted mask is desirable.
The present invention provides a method and apparatus for producing 0 degree light and 180 degree phase shifted light having substantially equal intensities as that light exits an alternating phase shifted mask. In the present invention, a material is inserted within the etched portion 112 of the 180 degree phase shift channel 114 of a reticle 100, wherein the material contains an index of refraction such that the first order light (+1, xe2x88x921) is propagated through the 180 degree channel 114. The end result is a 180 degree phase shifted light having an intensity substantially equal to that of the 0 degree light.